Again with my questions to fix my dear guitarrist issues with his
rack. I think I already have a clear picture on how to proceed, but if you mind take a
look into the novel I am about to write, and give me your opinions, I will greatly
appreciate the learning experience.

Chapter
1) The gear part.First part: The electrical instalation.The electricity
distribution starts with a mains transformer to get 110v (Argentina is 220V and my friend
got most of his gear in the US. Earth is OK (connects straight from the wall to the earth
terminal on the 110V side). This goes to the rack (an SKB unit) to a Furman power
conditioner. All rack units connect to the rack with 110V three terminal male plugs
(except one unit, more about this later)

I checked with a continuity tester and
all external housings show good continuity to the earth terminal on the 220V side of the
transformer (even the unit that does not have the earth connection). I did this test with
all the audio and midi cables connected... must recheck with all cables removed...

Second part: The modules.The rack contains the following items:SansAmp
PSA-1, Digitech IPS 33-B, Korg A2, Lexicon LXP-15 II, Behringer Denoiser NSR 2000 and
Behringer Ultrafex PRO EX 3200. To control all this gear he has a ground control
pedalboard "fueled" by a wall-wart transformer (that also goes to the Furman Unit) and
MIDI cables all over chaining everything.

Regarding the MIDI chain, despite
jumping from module to module instead of going through a MIDI patch bay, it is working
OK.

The Korg A2 still has the original 110v plug without earth... do you think
it worth changing the cable (or plug) to include an earth terminal on the "furman side"?
What if the cable does NOT have a third cable on it? Should I ask a technician to open up
the unit and fit a ground line? or just connecting the housing to any other rack unit will
be enough to be "safe"?

Also we have the chain of MIDI cables that (I think) will add more to the ground loop
potential!!

Chapter 3)
My approach to the solution and questions.First: Should I worry about the KORG unit
not being Earthed (as already mentioned up there). Suggestions to solve itSecond:
Should I worry about the units being bolted to the Rack housing hence creating a common
earth parallell to the cables?Third: I could not find any info on the Digitech and
Korg connections being Balanced or Unbalanced. I found an A2 service manual on the net,
and the schematic looks like both inputs and outputs are unbalanced. Lexicon outputs, no
clue (input says "unbalanced" on the manual!!). Is there an easy way to tell if an output
or input is balanced or unbalanced?Fourth: How can I check if the inputs and outputs
have "ground lifts"... I remember reading an article in SOS, but was unable to find it
again!!!! shame on me.Fifth: Can MIDI cables help in creating ground loops on the
audio side of the business? This MIDI cables are quite long and "looped" inside the rack
(nobody manufactures patch-length MIDI cables in Argentina!!!!)

My candidates
to the problems are all the unbalanced cable connections, and my idea was to strip all the
rack out, separate the parts and start installing one rack unit at a time, connect, check,
and move on adding the next module to the rack... Once detecting the "offending
connection" just cut the sleeve connection out and move on with my life. Just purchased a
sensible amount of good quality connectors and cable to do this.

And this leads
me to the final question. I have microphone cable... when soldering for unbalanced
connections, I do not know if soldering BOTH center cables to the tip makes a difference
vs connecting just one and leave the other "floating"....

I'll start with some more straightforward information and leave the hard work for others

The MIDI side should be the least of your problems. The MIDI spec requires
all MIDI inputs to be opto-isolated which goes a long way towards preventing ground loops
with MIDI cables. You might find this Hinton Instruments MIDI guide very useful as it gives you a lot of
detailed info in a very digestible form.

However, it might be worth checking
each of the MIDI devices to confirm which of the ins/outs/thrus are connected to the
chassis ground as that might come in useful later.

And while you are checking
the MIDI chassis grounds, you should also check the audio ins and outs to see which of
those are connected to the chassis as well.

Unfortunately, a lot of rackmount
guitar equipment is designed and built by companies that don't generally make rack
equipment. They test their gear in isolation but don't always consider whether it will
play nicely with other rack gear from other manufacturers. So if all audio ins & outs and
MIDI connectors are grounded through the chassis there is a lot of scope for ground loops
once you bolt them all together in a steel rack with steel bolts.

My starting
point would be to remove everything from the racks, lay it out so none of the cases have
any physical contact (side by side but not touching if you can, if not, stack them with
cardboard, wood, plastic sheet, or anything else non-conducting between them) and then
connect everything up so that the cables are the only connections. If you still get hum,
you have to use a process of elimination to work out where the problems lie and eliminate
them. Once you've done that start rebuilding the racks piece by piece and regularly
testing. If the hum returns when you rack the gear you might have to invest in nylon
washers/nuts&bolts to isolate some of the rack units from the rack case.

Again with my questions to fix my dear guitarrist issues with his rack. I think I
already have a clear picture on how to proceed, but if you mind take a look into the novel
I am about to write, and give me your opinions, I will greatly appreciate the learning
experience.

Chapter
1) The gear part.First part: The electrical instalation.The electricity
distribution starts with a mains transformer to get 110v (Argentina is 220V and my friend
got most of his gear in the US. Earth is OK (connects straight from the wall to the earth
terminal on the 110V side). This goes to the rack (an SKB unit) to a Furman power
conditioner. All rack units connect to the rack with 110V three terminal male plugs
(except one unit, more about this later)

I checked with a continuity tester and
all external housings show good continuity to the earth terminal on the 220V side of the
transformer (even the unit that does not have the earth connection).

Be careful using a Furman power conditioner
with that "mains transformer" (from your description, sounds as if it's really an
autotransformer, not a true step-down isolation trans):

Thanks for the input (a very cheap pun on words... but you know what I mean).

Regarding the transformer... YOU ARE RIGHT!! it is an autotransformer indeed. Before
plugging the transformer, I checked the wirings and had continuity across primary and
secondary windings, so before plugging the rack, I checked "polarity" on the 110V side and
the plug orientation was OK, so every rack unit received neutral through neutral, and
viceversa. Thanks for the advise!

Not
sure if I will be able to get those here in argentina... but I am thinking on how to
"clone" them.... lovely!

Thanks.

They are exactly what I had in mind when I suggested nylon nuts,
bolts, and washers. But as I wasn't sure whether or not you could easily get them in
Argentina I decided to avoid mentioning a specific product.

Part of the isolation is easy to replicate in a variety of ways (using the aforementioned
nylon washers front and back, or cutting your own strips of plastic to fold around, etc.)
but the crucial bit is the sleeving on the head end of the bolt, which has to be just the
right length to insulate the bolt from the rack without fouling the captive nut and strong
enough to not wear-through with the weight of equipment / vibration.

Plus, you
may need to add more insulation if either the faceplates or back the rack equipment is
likely to touch when installed. I find self adhesive neoprene tape or cut-up mousemat foam
to work well.

I agree with previous comments though - first layout all your
gear in an isolated fashion and then connect / add / remove pieces to establish where the
problem is coming from.

Yesterday night we had our first gig (as this
band) and everything went OK. The Sound Lady gave us a hand to take the noise out (without
cutting earth wires!) with DI boxes and some patch cables (seems that they cross with this
kind of issues all the time with amateur bands).

Now, the in-famous rack will
have a pit-stop in my home for a full diagnostic run and solving all earthing, looping and
stuff...

Regarding the rack isolation, I was thinking in line with the self
adhesive neoprene but with rubber (I have a rubber wholesaler not far from home) and find
out how to isolate the "exposed thread" part of the bolts (have an idea...).